The flow of 'emigration' would only be two-way initially. Saying that humans can survive in low gravity is not the same as having a good quality of life. We have no idea how it would affect the heart, the skeleton, muscles, psychology, fertility. I could go on but you get the picture. The psychological strain alone could make settling somewhere like the moon a non-starter. Why live in a tin can there when you're looking at a beautiful blue marble every day?